Human immunoglobulins contain two identical heavy chain polypeptides (each about 54 kilodaltons in MW) and two identical light chain polypeptides (each about 24 kilodaltons in molecular weight) which are bound together by disulfide bonds. Each light chain and each heavy chain include a constant region and a variable region. The variable region is located on the N-terminal portion of each chain and the constant region is located on the C-terminal portion of each chain. The constant regions of the light chains and heavy chains have different amino acid sequences, and can be used to identify the isotype of the heavy or light chain. In humans, there are two different isotypes of light chain polypeptides referred to as either kappa or lambda; and five different isotypes of heavy chain polypeptides referred to as gamma (IgG), alpha (IgA), mu (IgM), epsilon (IgE), and delta (IgD).
Clinical laboratories currently quantify and isotype serum immunoglobulins using a combination of protein gel electrophoresis (PEL) and imunogixation (IFE). For a normal healthy individual the electrophoretic pattern observed is an evenly dispersed staining pattern. This pattern reflects the polyclonal background produced by the large number (approximately 6.3×106 heavy chains and 3.5×105 light chains) of immunoglobulin heavy chains and light chains generated as a function of somatic hypermutation. In certain diseases, such as polyclonal gammopathy, there is an increase in the total amount of immunoglobulins in the bloodstream or in urine relative to a healthy individual. In other diseases, such as multiple myeloma, this increase in the amount immunoglobulins is due to a monoclonal immunoglobulin in the bloodstream. If high levels of the monoclonal immunoglobulin are detected, additional tests are performed to determine the isotypes of the heavy and light chains of the monoclonal immunoglobulin.
Likewise, clinical laboratories now assess cerebral spinal fluid (CSF) with isoelectric focusing gel electrophoresis followed by IgG immunoblotting (IgG IEF) to detect IgG clones in CSF as compared to serum. See e.g., Fortini A S, Sanders E L, Weinshenker B G, Katzmann J A. Am J Clin Pathol. 2003 November; 120(5):672-5. One or more CSF bands (i.e. oligoclonal bands; OCB) that are not present in serum suggest that B cell clones are actively producing IgG as part of an inflammatory response in the CNS. Detection of OCB is a sensitive method for CSF inflammatory diseases, and in MS 95% of patients have IgG CSF-specific OCB. Awad A, Hemmer B, Hartung H P, Kieseier B, Bennett J L, Stuve O. J Neuroimmunol. 2010 Feb. 26; 219(1-2):1-7.